Accepting parts off the machine tool is a rapidly growing trend in manufacturing as companies strive to make 100% accurate parts in the lowest possible cycle time. In order for the machining center to serve as an inspection instrument, it must be able to achieve the specified accuracies for the part produced. Most machining center users are familiar with laser calibration and CNC error compensation for systematic errors in linear and rotary axis motion. But relatively few realize the spindle-mounted probe can also work as an error-compensation tool.

A probing technique called artifact or reference comparison lets machining centers test their positioning accuracy against dimensional masters to determine corrective compensations. It's especially effective for angular and transient thermal errors, which are “two types of error that laser compensation can't address,” says Dave Bozich, product manager-Machine Tool Probes for Renishaw Inc., Hoffman Estates, Ill. “Artifact comparison can be used to enable almost any machining center to hold tolerances near its repeatability specification, as well as compensate for thermal effects,” he says.

The basic metrology technique uses a premeasured artifact as the reference master. To qualify as a master, the artifact's actual dimensions are measured on a CMM calibrated to a traceable standard. It's then located in the machining envelope, typically as part of the setup or fixture. It should be made of the same material as the parts being machined in order to ensure it responds identically to thermal changes. Probing the artifact before a critical machining pass, the CNC can check its own positioning against the known dimensions and program an offset to compensate for any discrepancy.

The technique is used by Renishaw in its own manufacturing to hold tight tolerances on small, precision components in an automated machining system. An artifact is indexed into the machining envelope as part of every batch of parts. At various points in the machining cycle, a spindle-mounted probe measures the size of a feature on the artifact for comparison against the known dimension. The CNC then compensates for machine scale, geometry, and thermal errors, enabling the machining systems to run unmanned for 140 hours per week.

Every machine has its own set of numerous small errors in motions and structure,” says Bozich. “As a result, there is always a slight discrepancy between a CNC's programmed position and the true position of the tool tip — even after laser compensation has brought the two into closer agreement. Programmable artifact probing provides a way to further compensate for remaining machine errors. It gives process control feedback to enable positioning accuracies that can approach the machine's repeatability spec.”